In 1771, King Louis XV made Claude-Nicolas Ledoux his Commissaire des Salines (commissioner of saltworks) for Northeastern France. It was essentially a political appointment, a touch of Ancien Régime patronage bestowed on Ledoux because he was the favorite architect of Madame du Barry, the king’s official mistress. Yet Ledoux was an idealist, a man of the Enlightenment. He was determined not just to oversee the salines but to reconceive them completely.

He set to work in the town of Chaux, proposing that the saltworks be arranged geometrically and designed according to neoclassical precepts. The king rejected his plan, especially objecting to his choice of classical columns more appropriate for a palace. So Ledoux started over. He invented his own order of pillars, shaped like stacked salt crystals, and changed the ground plan from square to semicircular.

Built in the mid-1770s, the new salines were the most stylish industrial architecture that the world had ever seen, and perhaps also the most rational. The director’s house was situated in the middle, flanked by two identical factory buildings, each supplied by its own woodshed to diminish the fire hazard. Workers were housed in separate buildings organized in a semicircle along the periphery—each provided with a vegetable garden—offering plenty of fresh air yet keeping them in sight of their supervisor. The saltworks were the architectural embodiment of a benevolent dictatorship.

However Ledoux’s planning didn’t end there. Even after the salines were complete and he was given the more prestigious job of erecting a wall around Paris—ordained by Louis XVI to collect royal duties—he continued to conceive improvements for Chaux. By the time the French Revolution ended the monarchy and his professional career, his scheme encompassed the whole factory town. Chaux was his Utopia, and the less chance he had of building it, the more grandiose were his dreams.

Ledoux published his vision in 1804, two years before his death, in a lavishly engraved book titled L’Architecture Considérée Sous le Rapport de l’Art, des Mœurs et de la Législation. The book included fully elaborated elevations and plans for entirely new types of buildings such as a Pacifère (a Temple of Peace where people could meet to resolve misunderstandings) and an Oïkéma (a Temple of Love, or brothel, where boys would be prepared for marriage). There were also private houses and public baths. Only a hospital was omitted, since people could be expected to live in perfect health in an ideal society perfectly ordered by Ledoux’s enlightened planning.

With L’Architecture, Ledoux pioneered a new kind of fiction, in some respects akin to utopian novels such as Francis Bacon’s New Atlantis and Thomas More’s Utopia, yet with a power that written stories lack. Utopian novels can merely describe the world as it might be. Visionary architecture, in contrast, provides a blueprint for making the hypothetical become reality. You might find the architect’s vision promising or threatening, but you cannot view it impassively. Architectural fiction is shovel-ready.

The most famous of architectural visions was conjured in 1919 by Vladimir Tatlin. His towering Monument to the Third International was intended to be nothing less than the hub of a future world government, the ultimate manifestation of the Bolshevik Revolution. The exterior was to be a 1,300-foot-tall steel spiral—the tallest building in the world—angled to match Earth’s axis. Inside metal girders representing the rising workers, Tatlin planned to suspend three glass rooms, turning in symbolic allusion to the Revolution. On the bottom was to be a cube housing the people’s congress, revolving once a year. Above it was to be a pyramid holding the party leadership, rotating once a month. And on top, a cylinder contained the propaganda ministry, capped with a hemispheric radio transmitter, spinning once every day. From inside the cylinder, the government would not only issue regular radio bulletins but also project the daily news against the night sky. (At least that part of the scheme was easily achieved. This being Russia, the sky was generally overcast.)

Tatlin didn’t just draw plans for his tower. He also made several models out of wood and wire. The first was more than 16 feet tall and loomed over the Eighth Soviet Congress in 1921. As Lenin called for the electrification of Russia, Tatlin’s monument stood as an emblem of the technological future. A second version, constructed for the 1925 Paris International Exhibition, peddled the glory of Communism to the world. A third, paraded through the streets of St. Petersburg during the 1925 May Day celebration, served as a unifying symbol, fortifying resolve as the planned economy failed.

At the same time, Tatlin’s tower was being debated within the party ranks, and the argument served as a surrogate for a larger argument about what direction the young country should take. One critic was Leon Trotsky, who questioned Tatlin’s decision to make the rooms rotate. “I cannot avoid asking: for what purpose?” he wrote in 1924. The sheer extravagance of Tatlin’s proposal forced Soviet leaders to consider whether their first obligation should be to stir up world revolution or to provide for their own people.

Like all ideals, the Monument to the Third International was purged by Stalin, and Tatlin was publicly criticized for his “formalist errors”. (He died of food poisoning in 1953.) Yet even the loss of his models and of all but a few original photographs did not occlude his vision, at least as seen from the West. In the summer of 1968, as Europe erupted in leftist revolt, the Moderna Museet reconstructed the model tower in Stockholm. In subsequent years, the tilted silhouette has become an icon of Communism’s unfulfilled promise nearly on par with the stenciled head of Che Guevara.

Undoubtedly the tower would have suffered a different fate had it actually been built. For architectural stability, the tilt would have had to be decreased, undermining the symbolism of the building: No longer would it have been the axis of world power. And surely the rotating rooms would have malfunctioned, every glitch made visible by Tatlin’s open-work Constructivism, generating inopportune metaphors as the government faltered. Built, Tatlin’s tower would have become a colossal joke (just as Ledoux’s Chaux would have been a public health disaster). Architectural fiction is a precarious genre. Visionary architecture must be viable enough to provoke concrete consideration while eluding physical implementation.

No modern civilization would be complete without architectural fiction. While ordinary architecture literally shapes the way in which people live, visionary architecture provides infrastructure for the collective imagination. Unrealized plans and models are meeting places for discussion. And if built architecture must be judged by practical criteria, such as whether it leaks or can withstand an earthquake, speculative architecture is only as good as the questions it raises.

Why not build a dome over Manhattan? That’s the question Buckminster Fuller posed in 1965 in a three-page special supplement to the St. Louis Post-Dispatch titled “The Case for a Domed City.” As the inventor of the geodesic dome—on which he held U.S. Patent #2,682,235—he may not have been the most disinterested party, but the arguments he made still resonate today.

The glass dome that Fuller proposed, first conceived in 1960, would easily have been the world’s biggest manmade structure. Spanning from the East River to the Hudson, and from 64th to 22nd Streets, it was to be two miles in diameter and a mile high at the center. “The peak of the Empire State building’s television tower would reach only a third of the distance from the street to the domed surface above it,” Fuller noted. (If erected in the Soviet Union, it could have accommodated a tower four times the height of Tatlin’s Monument.)

The geometry of the geodesic dome—in which forces are balanced by a system of interlocking triangles—sets no upper limit on the size at which one can be built. The structure grows proportionally stronger with increased diameter, requiring proportionally less material to hold it up. Already, Fuller had made one for the Union Tank Car Company in Baton Rouge that spanned 384 feet, at the time the largest freestanding structure in history. A dome over midtown New York would be 28 times that size, yet could be achieved, Fuller told Time magazine, with less steel than was contained in the Queen Mary.

The greater leap was conceptual. Even Fuller realized that domed cities would be a hard sell, and allowed that “the doming-over of established cities in moderate climate will probably not occur until domed-over cities in virgin lands have proved successful,” citing a dome over Antarctica as an example. “The established cities,” he continued, “will probably not adopt the doming until environmental and other emergencies make it imperative.” At that stage, it would no longer be about blissful living in an air-conditioned “Garden of Eden interior” but rather a matter of maximizing efficiency. The dome would make Manhattan a closed environment, facilitating recirculation of energy within while screening out pollution. Domed cities would become islands of survival on an uninhabitable planet. Fuller’s architectural vision was a premonition of a world wrecked by humans.

But he was an optimist. Aside from occasional references to nuclear fallout—from which the aluminum-coated shatterproof glass was supposed to shield residents—he seldom dwelled on apocalypse. Wiser use of resources—partially achieved by building family domes instead of inefficient brick houses—would save society from ruin, and the proposed dome over Manhattan would help to impart the required wisdom. A domed city would extend the psychological bubble in which people were accustomed to living—wastefully buying processed goods and throwing away whatever went unused—by extending the bubble to encompass an entire community. The full cycle of materials and energy would be manifest in a closed system, as would the reach of personal responsibility. In a sense, the dome over Manhattan was a miniature version of the ultimate closed system: spaceship Earth. Certainly it was less abstract. Simply imagining the glass dome, people could envision their extended presence on the planet.

Even by Fuller’s high standard, architectural fiction can be astonishingly efficient. The largest freestanding structure ever conceived required no more support than a sheet of paper.

Lebbeus Woods also had a vision for New York. The recently-deceased visionary architect—currently the subject of an absorbing retrospective at the San Francisco Museum of Modern Art—came up with a way of vastly expanding available real estate on the island by draining the East River and the Hudson, making the new community of “Lower Manhattan”.

In Woods’ 1999 architectural rendering, Lower Manhattan was literally situated below the city, cut into the exposed strata of granite. The scale of this proposed residential block made the skyscrapers above look lilliputian by comparison, which Woods said was his point. “It’s peeling back the surface to see what the planetary reality is,” he explained in a 2007 interview. “It’s about the relationship of the relatively small human scratchings on the surface of the earth compared to the earth itself.”

Woods has often been compared to Giambattista Piranesi, with some justification. Piranesi’s haunting 18th-century prison scenes anticipate many of Woods’ architectural drawings in both aesthetics and sensibility, and Woods is one of the few contemporary architects able to match Piranesi’s draftsmanship. Yet there’s an essential difference: Though Piranesi also trained as an architect, his prisons are not drawn in the pictorial language of architecture. They belong to the same illustrational tradition as scenes by Jan Brueghel. Woods, in contrast, often made his anxious visions as buildable as Ledoux’s saline utopia.

Woods was especially drawn to regions in turmoil. In the 1990s, he witnessed the siege of Sarajevo, leading to one of his most extensive bodies of work. Asked to design a reconstructed Electrical Management Building, he developed a new architectural idiom for the entire city. His provocative idea was that the built environment should retain some of the violence that the place had experienced. Ruins should not simply be bulldozed and replaced, but should become the basis for an uncomfortable transitional architecture afflicted with “scars” and “scabs.” There’d be a political dimension to these hybrids—both metaphorically and literally bridging future and past—a physical memory. Living in a scabbed city might lead to genuine rehabilitation. Naturally Sarajevo preferred a more cosmetic fix. None of Woods’ architecture, including the Electrical Management Building, was actually built.

His schemes also went unheeded in Berlin, where he designed labyrinthine architecture that awkwardly connected East and West when the Wall came down, and in San Francisco, where he developed a post-earthquake vernacular that used rubble as a construction material. Not that he expected employment. For most of his professional life, he proudly shunned commissions. As he explained in a 2008 interview, “What interests me is what the world would be like if we were free of conventional limits. Maybe I can show what could happen if we lived by a different set of rules.”

By actively embracing the role of visionary architect, Woods was able to push architectural fiction into territory inconceivable to Fuller, Tatlin and Ledoux. Their dreams may have been unrealistic, but they drew their plans believing their architecture would and should eventually be made. Harboring no such illusions, Woods was free to conjure plans that he knew were unfeasible, such as his fantastical “Aerial Paris,” which he claimed would be suspended above the old city by static electricity. More remarkably, he was able to explore architecture that he knew would be objectionable—to other people and to himself as well. (He designed a torture chamber so harrowing that Terry Gilliam swiped it for a scene in 12 Monkeys.) As Woods explained in his 2007 interview, his motivation as an architect was to ask “what if?”

Woods offered no answers. Architectural fiction is the most open-ended of narratives. Any eventuality can be explored by the architectural visionary, but the architect can only inscribe the opening lines. All of us have both the opportunity and obligation to decide how it ends by choosing what to do with the plans.